feat: add configurable account selection strategies

Refactor account selection into a strategy pattern with three options:
- Sticky: cache-optimized, stays on same account until rate-limited
- Round-robin: load-balanced, rotates every request
- Hybrid (default): smart distribution using health scores, token buckets, and LRU

The hybrid strategy uses multiple signals for optimal account selection:
health tracking for reliability, client-side token buckets for rate limiting,
and LRU freshness to prefer rested accounts.

Includes WebUI settings for strategy selection and unit tests.

Co-Authored-By: Claude <noreply@anthropic.com>

src/account-manager/strategies/trackers/index.js

@@ -0,0 +1,8 @@
+/**
+ * Trackers Index
+ *
+ * Exports all tracker classes for account selection strategies.
+ */
+
+export { HealthTracker } from './health-tracker.js';
+export { TokenBucketTracker } from './token-bucket-tracker.js';